If you use Cotypist on more than one Mac, each of them learns your writing separately. iCloud Sync pools that history, so the Mac you set up last week knows the same vocabulary as the one you have typed on for a year.
This page covers what travels, how it is encrypted, what it costs you in iCloud storage, and how to delete it again.
Sync carries the recorded texts that personalization stores — nothing else. If you have never turned the recording on, there is nothing to sync and the switch does nothing.
Each input is stored with the context it was written in, such as the app name and the text Cotypist read from the window you were writing in (provided you have granted Cotypist access to your screen).
These do not sync:
Your inputs are encrypted on your Mac before they are uploaded, with a key that only your own devices can use. Apple stores the result without being able to read it, and neither can we — the data passes through your personal iCloud account, not through any server of ours. We never see it, and we could not hand it over if we were asked to.
The key reaches your other Macs through iCloud Keychain, so sync needs it switched on, which in turn needs two-factor authentication on your Apple Account. Both are on by default for most people. If a Mac ever reports that inputs are waiting for their encryption key, this is the setting to check, under System Settings → your name → iCloud → Passwords & Keychain.
For the wider picture of what Cotypist can and cannot see, read the Privacy overview.
You can switch the recording off for individual apps and websites from the Cotypist menu bar icon. Nothing you type there is ever recorded.
Synced inputs count toward your iCloud storage, at roughly a few kilobytes each. The Labs pane shows the running total, so you always know what it costs. A large history built up over a year is typically a few tens of megabytes.
Deleting recorded texts in Cotypist's settings deletes them from iCloud and from your other Macs too, not just from the Mac you clicked on.
To clear the cloud copy without touching what each Mac already holds, use Remove iCloud Data… in the Labs pane. That empties your iCloud and switches sync off. Every Mac keeps the inputs it already has.
Here are a common failure modes and how to fix them:
If sync says it is up to date but inputs you expected are missing on one Mac, please get in touch.